1. Field of the Invention
The present invention relates to a method of producing a compound semiconductor wafer by making a compound semiconductor epitaxially grow on a monocrystalline substrate of a group-IV element semiconductor.
2. Description of the Prior Art
In the case of a compound semiconductor, the band gap or energy gap thereof can be optionally changed by modifying the kinds and proportions of the component elements thereof. Accordingly, for instance, a light-emitting diode can be formed using such a compound semiconductor so that it may emit any wave-length light in the range from red light to green light determined by the composition of the compound semiconductor used. Some kinds of compound semiconductors which are very high electron mobility are useful as ultra-high frequency devices, coming into the limelight recently.
The compound semiconductor mentioned above, however, has disadvantages in that its crystal production process requires complicated and highly sophisticated techniques and its material is expensive. As a result, the monocrystalline wafer thereof produced is very expensive, and also a large size wafer thereof is difficult to obtain owing to the above-mentioned limitations on the production techniques.
For instance, in the case of a light-emitting diode of gallium phosphide (GaP) which emits green light, it is commonly produced by forming an epitaxial layer of GaP on a GaP or gallium arsenide (GaAs) substrate by vapor-phase or liquid-phase epitaxy and then by forming a P-N junction in this epitaxial layer. However, the above-mentioned GaP or GaAS substrate is very expensive and cannot be made large in size, and therefore the product is also very expensive. In addition, formation of the epitaxial layer by the above process usually requires to maintain the substrate at a very high temperature, normally at about 900.degree. C. or more. However, such a high temperature of the substrate causes decomposition of, for instance, a III-V compound semiconductor and escape of a group-V element thereof, and therefore the compound produced becomes stoichimetrically incorrect. Because of this reason, it is required to provide, for instance, means for confining the group-V element in a high pressure region so as to prevent escape thereof, and consequently the production process of the compound semiconductor becomes very complicated.
Meanwhile, for the purpose of obtaining an inexpensive and large-size GaP wafer, efforts are being made to use, as a substrate, a group-IV element semiconductor, for instance, silicon (Si) or germanium (Ge) which can be easily made inexpensive, large in size and high in quality. However, in this case, difficulties are encountered in forming a high-quality crystalline layer on a substrate because the lattice constant, thermal expansion coefficient and the like of the substrate are different from those of an epitaxial growth of, for instance, GaP. In addition, the epitaxial growth layer formed on the substrate tends to mechanically peel off the substrate.
Besides, in the conventional method, it is required to heat the substrate to a high temperature during the epitaxial growth process as mentioned above, and therefore silicon or germanium in the substrate tends to diffuse in the epitaxial layer thereby deteriorating the electric characteristics thereof.